A laser diode operates in two distinct regions which includes a laser region and a LED region. As shown in FIG. 1, the laser region is characterized by a linear relationship between the laser diode drive current above a threshold current, and the output illuminative power. In the LED region, the relationship between the drive current and the output illuminative power is non-linear, and the change in output illuminative power is very small for a change in current, relative to the laser region of operation. A means of using laser diodes for exposure in laser printers, is to drive the laser diode with a digital to analog converter (DAC) which equally divides the current into 2.sup.N -1 values, where N equals the number of bits input to the DAC. Because the laser diode efficiency (which is measured as milliwatts output power, per unit milliamp of input current) is very small in the LED region, a large range of input signal code values results in very little change in exposure.
One type of laser printer uses an n bit (e.g., 12 bit) DAC which drives an exposure device (a He-Ne gas laser and acoustooptic modulator or laser diode). The exposure device then prints exposures upon photosensitive media such as film. An output lookup table is constructed, through which the data is mapped prior to digital to analog conversion. The lookup table is calculated to force a linear mapping from code value to film density.
In the high density region of the Density vs. Log Exposure (D LogE) film curve, the delta exposure for a constant delta density is much greater than in the low density region. As a result, the output lookup table maps a number of input code values to a single output code value in the high density or high exposure region. This results in an inefficient use of code values for exposure.
It is known that the threshold of visibility for a small change in luminance is relatively constant in the luminance region over which radiographs are viewed (see M I Sezan, K. Yip, S.J. Daly, "Uniform Perceptual Quantization: Applications to Digital Radiography," IEEE Transactions on Systems, Man, and Cybernetics, Vol. SMC-17, No. 4, Jul/Aug 1987, pp. 622-634). Because this delta luminance parameter is related by a constant factor to delta density, the optimum laser printer would use an exposure means which, in a monotonically increasing manner, maps each code value to a unique density value, and for which delta density is a constant. Thus, based upon the D LogE curve of the film, the optimum code value to exposure curve can be calculated.
The multimode behavior of the laser diode in the LED region, in addition to the low efficiency, make the LED region undesirable for exposure. In the LED region, the wavelength is lower than in the laser region, and much of the illuminative power is not at the fundamental mode of operation. Since the density of exposed and developed film is dependent upon the wavelength of exposure, the change in wavelength adds an additional source of error in the exposure process.
In U.S. Pat. No. 4,774,710, issued Sep. 27, 1988, inventors Davis and Hardy (see also U.S. Pat. No. 4,987,426, issued Jan. 22, 1991, inventors Ota et al.), it was suggested that at power levels below a cutoff point chosen to be in the laser region of operation, but just above the transition to the LED region, the laser be pulse width modulated to linearly extend the exposure to a minimum exposure level. The implementation of this idea requires that in the pulse width modulation region, the division of the pixel period be divided into pulse widths on the order of several nanoseconds. If a pixel is 182 nanoseconds, this implies the smallest pulse width be 1 nanosecond. At present this is not realizable in a cost effective manner with the state of the art in electronic components.
The following patents, which have also disclosed the use of multiple modulation modes in laser recording systems, have not been completely successful in maximizing the exposure range of the laser and in reusing the laser region of a laser diode. U.S. Pat. No. 4,799,069, issued Jan. 17, 1989, inventors Sasaki et al.; U.S. Pat. No. 4,806,946, issued Feb. 21, 1989, U.S. Pat. No. 4,375,065, issued Feb. 22, 1983, inventor Ohara; U.S. Pat. No. 4,905,022, issued Feb. 27, 1990, inventor Nagashawa; U.S. Pat. No. 4,754,291, issued Jun. 29, 1988, inventor Horikawa; U.S. Pat. No. 4,679,057, issued Jul. 7, 1987, for Laser Recording Apparatus.
Thus, there exists in known laser diode printer apparatus the problem of expanding the exposure range of the laser diode without operating in the LED region of the laser diode.